There has been known an ink cartridge including a remaining amount detecting portion having a prism. The remaining amount detecting portion is disposed in a storage chamber in which ink is stored. In the ink cartridge, it is determined, based on whether reflection of light emitted toward the remaining amount detecting portion from a light emitting portion has reached a light receiving portion, whether a remaining amount of ink stored in the storage chamber is low or not. The light emitting portion and the light receiving portion are disposed in a printer to which the ink cartridge is attached. In a state where the remaining amount of ink stored in the storage chamber is high and the remaining amount detecting portion is full of ink, in the ink cartridge, light emitted toward the remaining amount detecting portion is refracted at an ink layer in the remaining amount detecting portion and all is reflected off an inner wall of the remaining amount detecting portion. Therefore, no light reaches the light receiving portion. In a state where the remaining amount of ink stored in the storage chamber is low and the remaining amount detecting portion is empty of ink, light emitted toward the remaining amount detecting portion travels in straight lines in an air layer in the remaining amount detecting portion. Thus, the light reaches the light receiving portion.